The present invention relates generally to vacuum technology, and more particularly to novel methods and apparatus for cryogenic and other types of high vacuum pumping of unwanted gas molecules, including hydrogen, from an enclosed chamber.
Cryogenic vacuum pumps (also referred to as cryopumps) are widely used in high vacuum applications (10.sup.-6 to 10.sup.-11 Tort range), e,g., in semiconductor manufacturing Cryopumps are based on the principle of removing neutral gas molecules from a vacuum chamber by having the gas molecules lose enough of their incident kinetic energy by striking a cold surface so that the molecules remain bound to that surface by the dispersion forces of the cryogenic surface. Cryocondensation, cryosorption and cryotrapping are the basic mechanisms that can be involved in the operation of a cryopump. In cryocondensation gas molecules condense over previously condensed gas molecules thus forming a solid condensate. Thick layers of condensate form in pumping large quantities of gas necessitating the removal of the solid condensate during what is termed a regeneration or activation period. Basically regeneration is a process that releases and expels captured gases by warming and flushing the pump with dry and sometimes warmed inert gas.
Gases that are difficult to condense at normal cryopump operating temperatures can be pumped at higher temperatures by cryosorption. In this case, a sorbent material such as activated charcoal is bonded to the cryogenic surface. At cryogenic temperatures the binding energy between the gas molecules and the adsorbing particles is greater than the binding energy between the gas molecules themselves. This causes the gas molecules that cannot be condensed to adhere to the sorbent material and thus be removed from the vacuum system. When several monolayers of adsorbed gas have been built up, the effect of the adsorbing surface is lost and gas can no longer be pumped by this process until the absorbed gas molecules are removed. To remove the gas and reactivate the adsorbing surface, the sorbent material is heated to the point that the adsorbing particles lose their affinity for gas molecules. This releases the gas enabling it to be flushed from the pump chamber.
Cryotrapping can also be used to pump gases that are difficult to condense. In this case, the sorbent material that performs the cryotrapping is an easily condensable gas. The sorbent gas is admired into the pump, forming a condensate on the cold surfaces, The gases that are difficult to condense are also admitted to the pump at the same time as the sorbent gas and adsorbed on the newly formed surface of the easily condensable gas. A mixed condensate is thus formed. For example, if argon and hydrogen are present in a cryopump chamber, for every 1000 molecules of argon that condense, it is possible to trap one molecule of hydrogen in the argon.
Cryopumps are widely used for applications where contamination by non process gases such as hydrocarbons must be avoided. Cryopumps typically use a closed-loop refrigeration system with high-purity helium as the working medium. The refrigeration cycle involves the compression of gaseous helium by a compressor, removal of the heat of compression via a heat exchanger, filtering of the compressed helium, and subsequent two-stage expansion of the gas in a cryoexpander to produce the desired refrigeration. In one type of cryopump, metallic panels are mounted in an array on the first-stage expander within the pump. This cryoarray as it is called, when lowered to .apprxeq.50.degree. K. to 80.degree. K. by the refrigeration system, pumps water vapor molecules present within the pumping chamber and also functions as a radiation shield to the second stage array. A second stage cryoarray is usually mounted under the first stage and achieves a nominal temperature of 14.degree. K. to pump most other gas molecules. However, the lowest temperature achievable in a refrigerator cooled cryopump is only about 10.degree. K. so that not all gases normally present in a vacuum system can be pumped by cryocondensation. The gases which are difficult to condense in a vacuum system, such as hydrogen, helium and neon, must be pumped by other means, such as cryosorption. One commonly used technique to eliminate such gases uses a sorbent material, such as activated charcoal, permanently attached to the second stage cryoarray. Only relatively low amounts of gas can be pumped by cryosorption, as only a thin layer (up to about 5 monolayers) can be formed on the surfaces before the process ceases. To pump large amounts of gas, a large amount of sorbent material must be used in the pump. The underside of the cryoarray is usually coated with the sorbent material to cryoabsorb the non condensable gases. When the sorbent material becomes saturated, the system must be reactivated.
Unfortunately small particles of the activated charcoal can break off the surface of the cryoarray, migrate through the cryopump to the vacuum chamber and onto the surfaces of the work piece being processed thereby causing contamination. This problem is particularly acute in the manufacture of semiconductors where particles of almost any size are likely to produce defects in the end product. The use of ion pumps in conjunction with cryopumps to enhance cryopump performance is disclosed by Johann de Rijke in "Performance of a Cryopump-Ion Pump System," Journal of Vacuum Science and Technology, Vol. 15, No. 2, March/April 1978, Pp. 756-767. A standard cryopump with sorbent charcoal on the second stage and a standard noble gas ion pump were used to increase total pumping speed and total capacity of gas that can be pumped before regeneration was needed. The disclosed configuration did not address the problem of vacuum system contamination by charcoal particles.
A turbomolecular vacuum pump having a heat exchanger located in its suction port is disclosed in U.S. Pat. No. 4,926,648 issued May 22, 1990 to Okumura et al. The heat exchanger is connected to a refrigerator through a refrigerant pipe. The refrigerant is cooled from about -100.degree. C. to about -190.degree. C. and is used to condense water vapor.
Getter pumps are a very specialized type of noncryogenic pump. Such a pump functions through the chemical sorption of reactive gases that contact the surface of the getter material, such as an alloy of zirconium-vanadium-iron. The sorption of gas molecules by the getter material will continue until the surface layers become saturated, at which point the material must be reactivated by raising under a vacuum, the temperature of the getter material to several hundred .degree. C. As is well known heating causes the sorbed gas molecules to diffuse into the bulk of the getter material or, as is the case with H.sub.2, be driven off as a gas. Many active gases when sorbed into a getter act as "poisons" in the sense that they are irreversibly combined with the getter material thus limiting the useful life of the pump. Getter pumps are typically used for ultra-high vacuum pumping where a vacuum is needed for long periods of times, e.g,, in high energy particle accelerators or in the purification of low pressure rare gases. In applications where the vacuum chamber pressures vary considerably or where work cycles are encountered with new gases being periodically introduced, such as in the manufacture of semiconductors, getter pumps quickly lose their capacity to efficiently pump chemically active gases normally present. After about 40 work cycles the getter material becomes substantially degraded through irreversible sorption of the gases requiring replacement of the entire getter material, For this reason getter pumps have been restricted to special applications where they will not be exposed to large active gases that are irreversibly sorbed by the getter.
In a cryopump structure disclosed in U.S. Pat. No. 5,231,839, "Methods and Apparatus for Cryogenic Vacuum Pumping with Reduced Contamination" filed Nov. 27, 1991 by F. Engle and J. de Rijke, the charcoal sorbent material is eliminated to remove the potential contamination caused by charcoal flaking, In place of the charcoal an auxiliary pumping device, such as an ion pump, is employed to remove the difficult to condense gases, such as helium and neon. Cryocondensed argon assists in cryotrapping hydrogen molecules from the main vacuum chamber, A gate valve connecting the ion pump and the main cryogenic pump is closed during periods of high gas loading to prevent overloading of the ion pump. Although such a cryopump system is effective in removing helium and neon without the potential contaminate of charcoal, the system is primarily effective in removing hydrogen gas in applications where a large amount of condensible gas is also pumped, such as argon employed in sputtering applications. Unfortunately hydrogen also is a major contaminant in most semiconductor manufacturing processes.
It is a general object of the present invention to provide an improved structure for vacuum pumping an enclosed chamber and provide consistent vacuum pressures suitable for use in the manufacture of semiconductors.
It is another object of the present invention to provide a cleaner vacuum pumping environment wherein a potential contaminate of the vacuum chamber by a sorbent charcoal material is eliminated.
It is a further object of the present invention to provide an improved pumping apparatus and method for pumping an enclosed chamber containing among other gases, hydrogen.
It is a further object of the present invention to provide an improved pumping structure for pumping an enclosed chamber on a continuous basis independent of the application or vapor loading.